The present invention relates to a head positioning system which drives a magnetic head from a current track to a target track in the radial direction of a magnetic disk. More particularly, the present invention relates to a head positioning system for floppy disk drives employing a closed loop servo control for positioning the magnetic head.
In a conventional floppy disk drive, the magnetic head is driven to a target track of the floppy disk by an open-loop controlled step motor. For this reason, the track width is determined by the drive pitch of the step motor and is relatively wide enough to render negligible the effects of track eccentricity. The track eccentricity includes a first component caused by disk exchange, which has the same frequency as that of the disk rotation, and a second component caused by environmental change, such as temperature and humidity changes, which has twice the frequency of the disk rotation.
In order to increase the recording capacity of the floppy disk, the track density must be increased, i.e., the track width must be reduced. However, an open loop head positioning system cannot realize a drastic reduction in the track width since the possible amount of reduction of the drive pitch of the step motor is mechanically limited and since the track eccentricity will not be negligible at a high track density.
A head positioning system employing a closed loop servo for a floppy disk drive is proposed in United States Pat. No. 4,630,145. In this system, each of a plurality of tracks is divided into a plurality of sectors. The servo information is recorded at the leading position of each of the sectors. A magnetic head reads the servo information and supplies it to a closed loop servo controller. The controller enables a fine step motor to move the magnetic head radially to a position near the center line of a target track in accordance with the servo information. The fine step motor moves the magnetic head by a pitch which is narrower than the track width every time the servo information is obtained. Thus, a fine track following operation is carried out using only servo information obtained from the leading portion of each of the sectors.
The sampling time of the servo information is determined by the speed of disk rotation and the number of sectors, both of which are predetermined. Accordingly, the sampling time cannot be shortened and is too long to permit the magnetic head to follow the second component of the track eccentricity with a high degree of accuracy. As a result, the conventional head positioning system cannot accurately position the magnetic head at the center line of the target track and as a result the track density must remain relatively low.
In order to enable the magnetic head to follow the second component of the track eccentricity, the inventor of the present invention has proposed a head positioning system in U.S. patent application Ser. No. 921,514 filed on Oct. 22, 1986. The head positioning system includes a closed loop servo system having a compensation digital filter which generates a compensation signal whose frequency is synchronized with the second component of the track eccentricity. The head positioning system can position the magnetic head at the center line of the target track with a high degree of accuracy after a track seek operation.
However, the head positioning system requires a relatively long period of time to position the magnetic head at the center line of the target track when the second component of the eccentricity of the target track is large, i.e., a deviation of the magnetic head from the center line of the target track is large when the closed loop servo is started to operate. Further, when the deviation of the magnetic head is larger than a track width, the head positioning system is apt to position the magnetic head at a center line of an outer or inner adjacent track to the target track by mistake.